usbnet: handle remote wakeup asap

If usbnet is resumed by remote wakeup, generally there are
some packets comming to be handled, so allocate and submit
rx URBs in usbnet_resume to avoid delays introduced by tasklet.
Otherwise, usbnet may have been runtime suspended before the
usbnet_bh is executed to schedule Rx URBs.

Without the patch, usbnet can't recieve any packets from peer
in runtime suspend state if runtime PM is enabled and
autosuspend_delay is set as zero.

Signed-off-by: Ming Lei <ming.lei@canonical.com>
Acked-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>